Press Release

New features in multipolar heavy-fermion materials

November 10, 2017

Often, striking changes of physical properties can be made only by replacing a key element without changing the microscopic structure of a crystalline solid. For instance, SmB6 is a Kondo insulator, LaB6 is a superconductor, and CeB6 is a heavy-fermion metal but these compounds have the same cubic structure (as in the illustration). What is more intriguing is that researchers are still reporting new features ever since discoveries of those materials more than a half-century ago. SmB6 was found to be a topological insulator which shows a metallic surface state and an insulating bulk state (see also CPfS press release on 12th of Dec. 2016). CeB6 has been renowned for an intricate antiferromagnetism and antiferro (AF)-orbital orderings of Ce3+ ions, but low-energy ferromagnetic fluctuations have been found in the latest inelastic neutron scattering experiment [Nat. Mater.13, 682 (2014)].

In the meantime, as Ce is substituted by La, much more complicated phenomena related to the thermal and the field-induced evolution of AF-multipolar phases appear, and the lack of knowledge leading to an accurate phase diagram has resulted in a limited understanding about critical phenomena in Ce1-xLaxB6.

Recently, scientists of MPI-CPfS, TU-Dresden, and I. M. Frantsevich Institute in Ukraine have discovered sophisticated thermodynamic phase diagrams of Ce1‑xLaxB6. By analyzing specific heat capacity at ambient pressure down to 0.05 K, elastic neutron scattering, and magnetocaloric effect (MCE), precise phase boundaries have been found in the temperature (T) - external magnetic field (B) plane. Especially, a quantitative MCE analysis have played a decisive role in determining T = 0 magnetic phase boundaries and in providing a direct evidence of an unforeseen high-entropy regime at low-T, low-B. Finally, when T = 0 x-B phase diagram is compared with x- and B-dependent Sommerfeld coefficient γ0(x, B), which is a direct measure of an effective electron mass (m*), the maximum γ0,x ( B) on the T = 0 phase boundary was recognized on which quantum fluctuations between arbitrary competing order parameters are also maximized.

In this regard, the existence of a large positive correlation between m* and multipolar fluctuations in a heavy-fermion metal is implied by our results.